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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 62 (1978)

Issue: 9. (September)

First Page: 1684

Last Page: 1695

Title: Organic Sedimentation and Genesis of Petroleum in Mahakam Delta, Borneo

Author(s): A. Combaz (2), M. De Matharel (3)


The delta on the eastern coast of Kalimantan, Borneo, is a typical sedimentary-delta model for hydrocarbon accumulation. Because of a remarkable sedimentary continuity since the middle Miocene, three superimposed paleodeltas separated by two transgressive sequences are preserved. Several oil fields have been discovered in the area. Geochemical and microscopic studies of the organic material indicate a history of biochemical and catagenetic degradation, migration of the hydrocarbons generated, and their concentration in the sandstone reservoirs.

The organic material in the source rocks generally is of continental and vegetal origin. The oils studied are highly paraffinic, increase in gravity with depth, and have a very low sulfur content and a CPI close to 1. The oils of the two fields of Bekapai and Handil do not differ significantly, except that the degree of maturation of oil seems lower in Handil than in Bekapai. The characteristics of the source-rock chloroform extract are basically the same as those of the oils but the CPI is greater, between C25 and C29, and there is a higher proportion of alkanes in the extracts.

The isoprenoid spectra, however, are very similar in both families of products. As a result it is concluded that the accumulations are probably not from source rocks in the vicinity of the reservoirs but originate at greater depths. The hydrocarbons could have migrated vertically about 3,000 m, chiefly along the faults present at both Bekapai and Handil. This process also could provoke the segregation of oils of increasing gravity with depth. Future studies probably will help to resolve this problem.

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